1. Field of the Invention
This invention relates to an electronic detector for detecting faults, and particularly arc faults and persistent overload current faults, in a variable frequency ac power system.
2. Background Information
Ac power generation and distribution systems for aerospace, industrial and residential applications have historically been designed and optimized to operate with voltage sources of a fixed amplitude and frequency. Examples of this include the 60 Hz system used for power generation and distribution in the United States, a similar 50 Hz system in Europe, and the 400 Hz power distribution systems which are standard on most aircraft.
However, there is interest in developing variable frequency power generation and distribution systems for aircraft. This new approach would dispense with the variable-to-fixed frequency transmissions which are presently used to generate 400 Hz ac power on aircraft, with the intent of reducing costs, weight and volume. In exchange, all electrical loads on board the aircraft would be designed to operate over a range of ac source voltage frequencies; for example, from about 200 Hz to about 800 Hz.
A known technique for arc fault protection in 60 Hz ac power systems detects the random step changes in ac current caused by the striking of an arc. When a time attenuated accumulation of the step changes reaches a selected value, an arc signal is generated. This technique is not adaptable to the 400 Hz aircraft systems. First, the 400 Hz waveforms are much steeper than the 60 Hz waveforms making it more difficult to distinguish the normal current rise from the step increase caused by an arc. Secondly, and more importantly, aircraft electrical systems utilize small gauge wires which can be very long and therefore have substantial resistance. As a result, a load close to the generator can draw a normal current that far exceeds a fault current on the remote part of the wiring. Thus, a fault current may draw a couple of thousand amps if close to the generator but only perhaps 30 or 40 amps if in a remote section of the wiring. It is not practical, therefore, to rely on the magnitude of current in an aircraft electrical system to distinguish an arc fault. U.S. patent application Ser. No. 09/621,253 describes an arc fault detector suitable for aircraft ac electrical systems or other ac systems operating at higher frequencies such as 400 Hz that generates a cumulative sum of amounts by which the ac current in each most recent cyclic interval exceeds the current in the immediately preceding half cycle in absolute magnitude. An arc fault indication is generated when a time attenuated value of this cumulative sum reaches a selected level.
Up until now, fault algorithms for electronic fault detectors have been designed for use with ac power systems in which the operating frequency has been essentially fixed. There is a need, therefore, for an improved electronic fault detector, and a circuit breaker incorporating such an electronic fault detector, designed to operate with variable frequency ac systems.
This need and others are satisfied by the invention which is directed to an electronic fault detector for detecting faults in a variable frequency ac electrical system that comprises a current detector detecting the variable frequency ac current flowing in the variable frequency ac electrical power system, and processing means for processing half cycles of the variable frequency current for indications of a fault. In accordance with one aspect of the invention, the processing means processes the half cycles of ac current for indications of arc faults. In accordance with another aspect of the invention, the half cycles of ac current are processed for indications of persistent overload currents thereby providing a short delay protection function for the variable frequency ac system.
More particularly, the processing means of the detector includes measuring means measuring the magnitude and corresponding period of selected half cycles of the variable ac current, and signal generating means that generates a fault signal as a function of the magnitude and corresponding period of the half-cycle. The measuring means also includes means detecting zero crossings of the variable frequency ac current for determination of the periods of the half cycles. The measuring means can determine the magnitude and period of successive half cycles of the variable frequency ac current.
For arc fault detection, the signal generating means includes accumulation means generating a time attenuated accumulation of the difference between the magnitude of successive half cycles of the variable frequency ac current and the magnitude of the immediately proceeding half-cycle weighted by the period of the most recent half-cycle. It further includes means generating the arc fault signal when the time attenuated accumulation reaches a predetermined arc fault value. The signal generating means can include a threshold means for only accumulating the differences between the magnitude of successive half cycles following a half-cycle in which the magnitude exceeds a selected threshold. The differences are accumulated only for a selected time period after the selected threshold is exceeded.
For the short delay protection, the signal generating means comprises accumulating means generating a time attenuated accumulation of the absolute magnitudes of successive half cycles scaled by the period of the half-cycle, and means generating a delayed overload current fault or short delay signal when the time attenuated accumulation reaches a predetermined short delay value.
The invention also embraces circuit breakers incorporating such fault detectors.